Ultrasonic sealing or welding is a process that generates frictional heat to bond thermoplastic materials together. In rigid plastic bonding, a sacrificial feature is typically molded in to one of the mating parts to focus the clamp force and concentrate the development of frictional heat, facilitating the process. This feature is called an energy director.
When using the ultrasonic sealing process to bond thin films, such as pouches, it is not possible to create features in the film as it is manufactured differently than injection molded parts. For this reason, the feature or features required to focus the friction must be incorporated into the tools. The simplest form of ultrasonic tooling for film sealing is a single raised bead in the anvil and a flat horn (sonotrode).
This concept has been used for decades and is effective in generating a leak free seal in films. This simple concept, however, does have some drawbacks. The resulting bond is extremely narrow which concerns some end consumers and limits the strength of the bond. The highly-focused bonding area causes almost all of the film's sealant layer to be forced out of the bond area. This results in a bond only on the edge of the seal bead. And because of the extremely small surface area of this design, the process operating window is very narrow, and is highly sensitive to either too much or too little weld force and duration. The excessive compression that results from such a focused bonding area sometimes damages film structures.
There have been modifications made in order to try to improve upon the simple bead design for added seal strength. One example is illustrated in U.S. Pat. No. 9,278,481 to Hull. This designed disclosed in Hull uses an interlocking horn and anvil with a specific pattern. It consists of a typical 90-degree peak, 4-sided pyramid that has been truncated, producing V-shaped grooves that run at 45 degree angles to the width of the package. The truncated knurl results in a slight engagement when the tools are interlocked. This offers sealing of greater surface areas and the processing window is widened.
However, this design has several drawbacks. In particular, the tools are intended to interlock with just a few thousandths of engagement which results in alignment that is critical and difficult to maintain. The pattern also consists of many sharp corners which stress films and result in tensile and burst failures at the leading edge of the seal. It also incorporates pyramid shaped pockets to accommodate the sealant layer that is forced from the bond area. Because these pockets have a sharp angle at the peaks, the film often bursts at these points from the pressure of the sealant flow overstretching the film in the peaks and valleys. This phenomenon, referred to as film fracturing, can result from the use of such designs.
U.S. Pat. No. 5,076,040 to Davis discloses seal bars for a pouch packing machine having asymmetric sealing surfaces. There are two opposing sealing surfaces with wave patterns defining ridges and grooves. Apices of the ridges contact nadirs of the grooves and at points between each ridge apex and groove nadir a space is formed. However, because the ridges and grooves are each curved and the convex surfaces have a radius of curvature that is less than that of the concave surfaces, the contact surfaces are minimal, which could lead to localized stress and strain.
Therefore, there is a need for improved ultrasonic sealing tools and processes.